The Cranston School Committee has voted to keep a prayer banner on public display at a city high school, even if it means a costly legal battle. The committee voted 4 to 3 Monday to keep the banner at Cranston West that starts “Our Heavenly Father . . .’’ and ends with “Amen’’ even though the American Civil Liberties Union has said it violates the Constitution’s principle of separation of church and state. The banner, which urges students to do their best, has been hanging in the school since the late 1950s. Students are not required to recite it.